MS is a disease which affects significant areas of the central nervous system required for normal neurological functioning. Pathological studies of patients have suggested a viral etiology for this disease, based on similarities to known viral illnesses and the pathological changes ensued. Epidemiological studies have also supported the theory of a viral cause of MS with reported outbreaks of the disease in areas such as the Faroe Islands. We now believe the our laboratory at the Rockefeller University, as well as other research labs have sufficient preliminary evidence to conduct a clinical trial in MS using an anti-herpes antibiotic, Valtrex, to determine if the ravages of this disease can be controlled. Specifically, we have demonstrated that the Humanherpes type -6 (HHV-6) virus can be detected in the brain specimens of MS patients (oligodendrocyte glial cells), but not in control brain specimens from other diseases. In addition HHV-6 DNA has been detected in MS brain tissue by the PCR technique, but not in controls. Finally, the serum of MS patients has demonstrated a high immune response to the Herpes-6 virus. Valtrex is an FDA approved drug inhibitory to the HHV-6 virus and a good safety profile. The general plan of study is to enroll 60 patients into a two year trial. Thirty patients will receive the drug, and thirty will receive placebo. The patients will be evaluated over the trial period by two neurologists, one blinded, to assess if the disease has progressed by causing more disability, as measured in disability rating scales. In addition the frequency of attacks will be monitored in the hope that those on Valtrex will experience fewer and less severe losses of neurological function over the two year period. Serial brain M.R.I. scans will be obtained to determine if those on the Valtrex have fewer brain lesions associated with MS over time. Blood and spinal fluid will be periodically drawn to determine the immune response to the HHV-6 virus in the Valtrex study patients and in patients not in the drug trial, but followed at Rockefeller under the protocol "Immunological Studies in MS."